Fix parallel build with Fortran modules.

This PR takes care of the parallel build issue noted in #30 by doing the following:
 * Use object files for the dependencies between modules. Using modules themselves does not work correctly.
 * Remove the unnecessary recipe for building the modules. They're already built by the default rules, and these were wrong anyway (they ignored `FCFLAGS`).
 * Add an extra dependency for the modules on the object files. This isn't strictly right because the modules are created from the source file, not the object file. However, there are no users of the module as target _dependencies_, so it makes little difference. The only reason these are added is so that `automake`'s `install` rule works correctly.